Rotating magnetic disks of the type found in disk drives utilize a magnetic recording head to read and write the information upon the disk. Typically, the magnetic recording disk is made from a substrate of aluminum having a thin film metal alloy, or other media which acts as the magnetic layer, deposited thereon. A protective layer of carbon is normally then deposited upon this magnetic layer. The carbon layer is used to protect the magnetic layer from wear and corrosion.
The magnetic recording head which is used to read and write the information upon the disk is supported upon a slider. The slider is in turn mounted upon a support arm. The slider itself is typically made of ferrite or a ceramic material such as alumina-titanium carbide or calcium titanate.
When not in use, the slider rests upon the surface of the magnetic disk. During information retrieval and recording, however, the magnetic disk is rotated. When the disk first begins rotating, the slider slides along the surface of the magnetic disk. As the rotational speed of the disk increases, however, a boundary layer of air is formed which causes the slider to lift off of the disk and "fly" above the surface of the disk. The distance of the slider from the disk during flight is approximately 100 nm. When the power to the disk drive is once again shut off, the disk rotational speed gradually decreases, and the slider lands upon the disk, sliding along the surface of the disk until the disk comes to rest.
Several problems arise from the contact of the slider with the disk. First, during start up and slow down of the disk, the slider is sliding directly in contact with the disk surface. This frictional contact causes wear of the disk and slider. The wear of the disk occurs, even though a protective carbon coating is applied to the disk, because of the extreme hardness of the slider material. The excessive wear on the disk reduces the effective useful life of the disk.
Contact between the slider and disk also occurs occasionally when the disk is at full rotational speed. Although the boundary layer of air normally acts to support the slider above the disk, high points (asperities) on the otherwise smooth surface of the disk often cause the slider to make contact with these projections on the disk. When the slider impacts these asperities on the disk, the slider often gouges the disk surface, further degrading the disk surface, as well as causing damage to the head and slider.
Several additional problems, other than wear of the disk and slider, have been identified with the use of prior art sliders. First, during initial rotational start up of the disk, friction between the slider and disk increases the force necessary to rotate the disk. This necessitates the use of large motors to turn the disks. Second, the magnetic poles of the head, made of an alloy of nickel and iron, which are exposed to the atmosphere are sometimes prone to corrosion. This is especially true in the case of magnetoresistive (MR) heads, where the MR sensor is particularly prone to corrosion. The carbon overcoat serves to protect the sensor from exposure to the environment. The corrosion of the magnetic poles and the magnetoresistive sensor interferes with the ability of the transducer to properly read and write the information on the disk.
Recently, one attempt was made to resolve some of the above identified problems. U.S. Pat. No. 5,151,294 discloses a method for forming a protective film of carbon upon the air bearing surface of a slider. This patent discloses a method of frictional carbon transfer wherein some of the protective carbon on a disk is transferred to a slider by starting and stopping the rotation of a disk so that the slider remains in contact with the disk during rotation. As a result of the frictional interference between the disk and slider during the many start/stop cycles, carbon from the overcoat on the disk is transferred to the slider surface.
This prior art method suffers from several disadvantages and incompletely solves the problems described above. First, when the carbon is transferred from the disk to the slider, the placement of the carbon on the slider is random. The random placement occurs because as the carbon transfer takes place, the carbon first begins adhering to the slider in raised spots (as on a microscopic level the slider is not absolutely flat) and in areas of high friction. Once the carbon begins to adhere to the slider in these areas, the remaining portion of the slider surface is then separated from the disk by the thickness of the transferred carbon. This means that carbon is deposited in certain areas of the slider, but not others. Utilizing this prior art method, the air bearing surface of the slider does not become completely covered, exposing the non-covered areas of the slider to corrosive agents. Further, the frictional method of carbon placement does not allow the carbon to be placed in specific thicknesses, nor in specific regions on the slider, as the placement of the carbon is purely random.
Other references, U.S. Pat. No. 5,159,508 to Grill, et al., and U.S. Pat. No. 5,175,658 to Chang, et al., describe the use of a DC biased substrate in an RF plasma deposition apparatus to deposit an adhesion layer and a thin layer of carbon upon the air bearing surface of a slider. These references describe depositing an adhesion layer to a thickness of between 10 and 50 angstroms (i.e., 1 to 5 nm), and a carbon layer to a thickness of 50-1000 angstroms (i.e., 5 to 100 nm) upon the flat surface of a slider. An etching technique is then used to form a patterned area, which includes rails, on the air bearing surface. A solvent is then used to remove the photoresist layer which is used to control the etching.
This prior art method suffers from several disadvantages as well. Primarily, the Grill and Chang references disclose a method by which the protective coating (plus a masking layer as described in the Chang reference), is placed to protect the slider. These layers are necessary to protect the slider during subsequent etching which is done to form the patterned air bearing surface, and from subsequent solvent removal of the photoresist layer after etching.
Unfortunately, this method requires, for practical purposes, the placement of a substantial thickness of coating across the entire slider in order that the sensor not be damaged during the etching process. Further, this method does not allow control over the depth of the coating material across the air bearing surface of the slider. In particular, during the etching and solvent removal steps, which are done to form the patterned surface in the slider and remove a photoresist material, the coating is removed in an uncontrolled fashion. This causes the coating thickness to vary across the air bearing surface of the slider.
Prior to this time, it has also been found extremely difficult to deposit a carbon coating utilizing a sputter technique to a thickness of less than or equal to 20-25 nm with any consistency. The difficulty arises from the fact that when the carbon is so thin, it is not a continuous film, and therefore adheres poorly to the base or substrate material. Very thick carbon films are unacceptable in the context of use with sliders because the placement of any coating upon the air-bearing surface of the slider further separates (remembering that during "flight" the slider is separated from the disk by an air space of as much as 100 nm) the head from the disk during read/write operations. The added separation of 25 nm caused by the addition of the carbon coating by the prior art methods is unacceptable as such a large separation interferes with the ability of the read/write head to operate effectively.
It is noted that the shape of the air bearing surface is important for it determines, at least in part, the flight characteristics of the slider. Prior to this time, the shape imparted upon the air bearing surface of the slider has been made by lapping the surface of the slider. This is a time and labor-consuming method, and is not very exact. In fact, current lapping techniques have a tolerance of only .+-.0.6 micro inches ( i.e., about 15 nm).
The prior art methods of coating placement do not allow selective placement and control over the shape of the coating material. Therefore, any shape imposed upon the air bearing surface of the slider must be done with the lapping technique before the thickness of coating is added with the prior art methods.